1. Field of the Invention
This invention relates to a power transmission system for four-wheel drive vehicle, specifically to a power transmission system which adjusts the distribution of power to the front and rear wheels of a vehicle according to the driving state of the vehicle.
2. Description of the Related Art
In recent years, electronic controlled on-demand four-wheel drive vehicles are in practical use widely. The related art is disclosed in Japanese Unexamined Patent Publication No. 2002-127772 (US2002/0055416A1), for example.
The four-wheel drive vehicle disclosed in this patent publication is based on a front-engine/front-drive vehicle, and arranged to supply power from an engine to the front wheels via an automatic transmission and supply part of the power from the engine to the rear wheels via a transfer clutch.
The amount of power transmitted to the rear wheels is adjusted by controlling the engaging force of the transfer clutch. By this, the distribution of power between the front wheels and the rear wheels is changed to achieve behaviors of the vehicle suitable for its driving state, such as ease of turning at an early stage of steering, restriction of slip in rapid acceleration, and stability in deceleration.
However, since the power transmission system disclosed in the above patent publication can adjust only the distribution of power between the front wheels and the rear wheels, there is a limit in controlling the turning characteristics of the vehicle, specifically in controlling the understeer and oversteer of the vehicle. Hence, for this control, a braking force control device, a traction control device, etc. need to be used together, which leads to complicated overall system structure and control, and hence problems such as increase in vehicle weight and significant increase in production cost.